<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div{
            width: 100px;
            height: 100px;
            background-color: red;
            display: none;
        }
        span{
            display: inline;
        }
        input:focus + div{
            display: block;
        }
        #box{
            width: 100px;
            height: 100px;
            background-color: #fc3;
        }
    </style>
</head>
<body>
    <!-- 
        块元素block
            div,h1-h6,p,ul,li
        内联元素inline
            span,a,i
        内联块inline-block
            img tr td
        none
     -->
     <div>1</div>
     <div>2</div>
     <div>3</div>

     <input type="text">
     <div id="box"></div>
</body>
</html>